Gold alloy with improved hardness

ABSTRACT

The invention concerns a gold alloy. 
     It comprises at least 75% of gold, 0.5% to 2.1% of aluminium able to form precipitates with gold, an additional metal able to favour a stable face centred cubic structure and able to increase the solubility of aluminium in gold, and a precipitate selected to obtain a hardness of more than 250 HV. The selected precipitate of aluminium with gold is the aluminium and gold precipitate Al 2 Au 5 . 
     It includes 0.5% to 2.1% of aluminium and a complement of additional metal including a majority of silver. 
     The method of obtaining this alloy regulates the controlled growth of the precipitate during a structuring tempering treatment following dilution and hardening. 
     The invention concerns the use of the aluminium and gold precipitate Al 2 Au 5  for hardening a gold alloy. 
     The invention concerns a timepiece or piece of jewellery including at least one component made of this type of alloy.

FIELD OF THE INVENTION

The invention concerns a gold-based alloy with improved hardness.

The invention also concerns a method of obtaining a gold-based alloy with improved hardness.

The invention further concerns the use of a precipitate for hardening a gold alloy.

The invention further concerns a timepiece or piece of jewellery including at least one component made of this type of alloy.

It is an object of the invention to make a gold-based alloy, which has improved hardness qualities compared, not only to pure gold, but also to known gold-based alloys.

The main applications are horology, jewellery and dentistry.

BACKGROUND OF THE INVENTION

Hardening gold is an old problem, which, since ancient times, has led to the use of alloys in order to obtain sufficient mechanical features to ensure at least the resistance of the manufactured articles. Indeed, the method of cold working material by plastic deformation, which is well suited to some metals, is ill suited to gold because gold has very little consolidation during deformation, and also recrystallises at relatively low temperatures. The grain size refinement method, which theoretically raises the elastic limit of the material, is not suitable for gold either, as gold has a face centred cubic structure, called “FCC” hereinafter, since there are enough active sliding systems for the free passage of dislocations from one grain to another.

Solution heat treating alloy elements is the method most commonly used, often empirically, and at best, it provides only mediocre hardness, of around 150 to 155 HV on the Vickers scale.

Various attempts have been made, for example to refine grain size as in EP Patent No. 0 284 699 in the name of Steinemann with a binary alloy containing gold and another metal chosen from among aluminium, gallium or silicon, or even with a similar pseudo-binary alloy also including copper, with a maximum gold concentration of 15%. This type of compound gives a centred cubic structure and a grain size of less than 50 microns, which provides some ductility, which is not the quality desired here.

The preparation of dental alloys with hardness which increases over time and at body temperature is also known, from U.S. Pat. No. 5,338,378 in the name of Kyushu University, which implements an alloy with 67% to 82% of gold, 18% to 33% of copper and 2% to 8% of at least one other metal selected from among gallium, aluminium and zinc. This alloy undergoes a hardening operation after being heated to between 650° C. and 700° C. prior to use. Likewise, EP Patent No. 0 978 572 in the name of Hafner GmbH discloses an alloy comprising 70% to 80% of gold, 15% to 25% of copper, 0% to 15% of silver and 0.1% to 5% of gallium, which, in an unexplained way, barely oxides during a second treatment at 400° C. which follows a first treatment at 800° C. and acquires a hardness which increases with time at ambient temperature.

The following documents are also known:

The document referred to as D1: JP Patent Application No 8 013060 A in the name of PILOT PEN discloses a method of obtaining a gold-based alloy with improved hardness and several possible compositions:

15% to 19% of copper and 4 to 10% of silver, and 0.3 to 1% of aluminium and/or magnesium.

or 15 to 19% of copper, 4 to 10% of silver, 0.3 to 1% of aluminium and/or magnesium, and 0.3 to 2% of zinc.

0.1% to 1% of ruthenium and/or cobalt is incorporated into this mixture. However, this composition corresponds to a rose gold, which is not claimed here.

The document referred to as D2: EP Patent Application No. 0 978 5762 A1 in the name of HAFNER discloses an alloy with 70 to 80% of gold and 15 to 25% of copper, to which 0.1 to 5% of gallium is added. According to some variants it may also contain: 0.1% to 3% of zinc, and/or 0.5 to 5% of silver, and/or 0.1 to 0.5% of silicon and/or 0.1 to 2% of iron and/or 0.1 to 0.3% of indium, and/or 0.1 to 0.5% of aluminium and/or 0.1 to 3% of tin.

The document referred to as D3: US Patent Application No. 5 38 378 A in the name of OHTA MICHIO discloses a dental gold alloy which ages slowly for 20 to 30 days during which its hardness increases further. It includes 82 to 67% of gold, 18 to 33% of copper, and 0 to 2% of a hardening accelerator chosen from between gallium and zinc. The alloy is heated to between 650 and 700° C. and water quenched. In another composition, it includes 2 to 8% of this type of accelerator which includes at least one metal chosen from among the group consisting of 1 to 4% of gallium, 0.4 to 2% of aluminium and 1 to 5% of zinc. In another composition, it includes 1 to 4% of gallium. In another composition it includes 1 to 5% of zinc.

The document referred to as D4: The article “18 carat yellow gold alloys with increased hardness” by SUSS, RAINER, published in 2004, discloses gold alloys with increased hardness and the influence of the additional metals, both as regards obtaining physical characteristics and particular colours.

The document referred to as D5: The article “Metallurgy of gold” by FISCHER-BÜHNER, published on 20 May 2010, details the metallurgy of gold and its alloys.

It has been shown that, from all the many studies made, it is known how to select gold alloys with suitable hardness, without always understanding the physico-chemical mechanism which infallibly results in the desired hardness.

In short, the known methods are often empirical, are not well grasped and create alloys which, on the one hand, only have average hardness, and on the other hand have a very particular colouring which is very different from that of pure gold.

SUMMARY OF THE INVENTION

The invention is concerned with developing an alloy which has good hardness properties, greater than 250 HV on the Vickers scale, and equivalent core hardness properties compared to hardness of around 155 HV currently obtained by solution heat treating metallic alloy elements.

It is also important for the appearance of gold and its brilliance to be preserved.

The invention therefore concerns a gold-based alloy, known as 3N 18 carat yellow gold, characterized in that it is formed of a mixture comprising in weight percent:

at least 75% of gold,

from 0.5% to 2.1% of a second metal chosen for its ability to form precipitates with gold, said second metal being aluminium, 20% to 25%, or preferably 22.4% to 24.5% of at least one additional metal chosen for its ability to favour a high temperature stable FCC structure, and for its ability to increase the solubility of said second metal in gold, and to adjust the colour of the alloy,

0% to 0.5% of one or more components selected for the fluidity and grain size refinement of said alloy, said mixture further including at least one said precipitate from the reaction between said second metal and gold, selected from among said precipitates of said second metal with gold to form an intermetallic compound providing said alloy with more than 250 HV hardness so as to improve the structural hardening of said alloy, said selected precipitate being the aluminium and gold precipitate Al₂Au₅.

According to another feature of the invention, said additional metal is silver.

According to yet another feature of the invention, said additional metal is silver and it is completed by another additional metal in a lower concentration than that of silver.

According to a particular feature of the invention, said other additional metal is copper.

The invention further concerns a method for obtaining a gold-based alloy with improved hardness, characterized in that:

-   -   a second metal is chosen for its ability to form precipitates         with gold, said second metal being aluminium;     -   at least one additional metal is chosen for its ability to         favour a stable FCC structure on the one hand, and for its         ability to increase the high temperature solubility of said         second metal in gold on the other hand;     -   the conditions are created for inserting precipitates of said         second metal with gold into an FCC structure resulting from the         solution heat treatment of a mixture of gold, said second metal         and said at least one additional metal;     -   a mixture is prepared comprising in weight percent:         -   at least 75% of gold,         -   0.5% to 2.1% of said second metal,         -   20% to 25% of preferably from 22.4% to 24.5% of at least one             said additional metal,         -   0% to 0.5% of one or more components selected for the             fluidity and grain size refinement of said alloy,             said second metal and said additional metal being selected             to obtain from among said precipitates of said second metal             with gold, at least one said particular precipitate of said             second metal with gold to form an intermetallic compound             giving said alloy a hardness of more than 250 HV;     -   said mixture is solution heat treated by being heated to between         650° C. and 700° C.;     -   after said solution heat treatment, rapid cooling is performed         in the form of ambient temperature hardening;     -   after said rapid cooling, a tempering structuring treatment is         carried out at a temperature of between 200° C. and 250° C. to         create said at least one selected precipitate from said second         metal with gold which is the aluminium and gold precipitate         Al2Au5;     -   said at least one selected precipitate is grown in a controlled         manner by maintaining said tempering structuring treatment for a         sufficient period of time, at least 60 minutes, to obtain the         desired hardness;     -   cooling at ambient temperature is carried out.

According to a feature of the invention, the selection of said at least one selected precipitate is limited to a single precipitate.

According to another feature of the invention, said tempering structuring treatment is performed at least 24 hours after said rapid cooling.

According to another feature of the invention, silver is chosen as said additional metal.

According to another feature of the invention, silver is chosen as said additional metal and another additional metal is added in a lower concentration than the silver.

In particular, copper is chosen as said other additional metal.

The invention further concerns the use of a precipitate for hardening a gold alloy, characterized in that said precipitate is an aluminium and gold precipitate Al₂Au₅ for hardening a gold alloy comprising at least 75% of gold, 0.5% to 2.1% of aluminium and 20% to 25% of at least one additional metal, chosen from among silver and copper for its ability to favour a stable FCC structure on the one hand, and for its ability to increase the solubility of aluminium in gold on the other hand, and 0% to 0.5% of one or more components selected for the fluidity and grain size refinement of said gold alloy, said use resulting from the insertion of said aluminium and gold precipitate Al₂Au₅ into an FCC structure resulting from the solution heat treatment of said mixture comprising gold, aluminium, said at least one additional metal, said selected components if said alloy has any, and aluminium and gold precipitates, said insertion being achievable via this method.

The invention further concerns a timepiece, piece of jewellery or dental piece including at least one component made of an alloy of this type.

In the preferred embodiment of the description, concerning an 18 carat alloy, the alloy preserves the specific appearance of pure gold. Because of its increased hardness, the alloy obtained is more scratch resistant and is entirely suitable for timepiece and pieces of jewellery and in particular for the visible components thereof such as bezels and middle parts of watches, and jewellery structures, bracelets, clasps, buckles and other items.

The invention provides a method that is simple to implement and reproduce, and which reliably produces a gold alloy with the required hardness of more than 250 HV in a short treatment time. The alloy obtained can be used immediately, without requiring any additional ageing.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will appear upon reading the following description, with reference to the annexed drawings, in which:

FIG. 1 is a diagram of phases of a pseudo-binary alloy Au—Ag—Al according to the invention, in the example of an 18 carat alloy, which represents the various phases according, on the x axis, to the concentration of aluminium, i.e. the ratio between the aluminium mass and the total alloy mass, and on the y axis, to temperature, shown here in degrees Celsius.

FIG. 2 is a diagram of Vickers hardness on the y axis according to time on the x axis, of an alloy according to the invention made within a preferred domain A of the diagram of FIG. 1 compared to an 18 carat gold obtained by a standard method.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

It is an object of the invention to make a gold-based alloy, which has improved hardness qualities compared, not only to pure gold, but also to known gold-based alloys.

The invention implements a structural hardening method, by the selection of particular elements, which are chosen here to form particular precipitates. Among the various precipitates which gold can form with other metals in very particular physico-chemical conditions, those precipitates whose germination and growth can be controlled, by implementing an appropriate treatment, to optimise mechanical characteristics, and in particular here to increase hardness, should be chosen.

In particular, the mechanical feature which the present invention improves, by creating a particular method, is hardness which concerns both the core hardness of the alloy, and surface hardness which is very important in horology and jewellery to ensure scratch resistance or at least to minimise the effects of scratches.

This is therefore a very different case to most gold alloys used in jewellery, which are usually developed to include the minimum gold content required to ensure an appearance close to that of gold, and with the requirement for a high level of formability, so as to allow lamination, or stretching, in hollow bodies or sheets, which are easy to shape and weld.

The inventive step of the invention has consisted in researching the possibility of inserting precipitates into a face centred cubic structure or FCC, and letting them grow in a controlled manner, so as to obtain a greater hardness than the usual hardness

The invention more particularly concerns the field of gold alloys with a high gold content, and more specifically, 18 carat alloys comprising at least 75% of gold content by weight.

The selection of aluminium is peculiar to the invention because of the ability of this metal to form different precipitates with gold: Al₂Au₅, AlAu₂, Al Au. These three precipitates can produce alloys with improved hardness.

These alloys Au/Ag/Al/ Al₂Au₅ or Au/Cu/Al/ Al₂Au₅ or Au/Ag/Cu/Al/ Al₂Au₅ do not exist in the natural state and they have to be made in order to be used. The invention proposes a preferred manufacturing method below.

It is preferable to carry out the method so as to obtain the Al₂Au₅ precipitate, which when incorporated in an alloy provides normal resistance during machining or transformation operations. Thus, this Al₂Au₅ precipitate is created and preferably only this Al₂Au₅ precipitate is created since it yields better properties than the other two precipitates AlAu₂ and Al Au.

The Al₂Au₅ precipitate must be obtained at the core of the FCC structure. A binary alloy formed solely of gold and aluminium is difficult to work and is very fragile, which makes it unsuitable for most jewellery uses. It is therefore necessary to stabilise the FCC phase by incorporating at least one other alloy element to ensure the solubility of the intermetallic compound at a high temperature, and also to ensure the longest possible FCC phase, i.e. for the broadest possible range of aluminium content. High temperature stability means that only the FCC phase is present in the temperature range concerned, which can be seen along the elongated single domain of the equilibrium diagram of FIG. 1.

Different trial pseudo-binary alloys have been tested.

The second metal can be chosen from among aluminium, silver, chromium, copper, iron, hafnium, manganese, niobium, palladium, platinum and vanadium although this list is not exhaustive.

An additional metal may preferably be chosen from among silver, aluminium, chromium, copper, iron, hafnium, manganese, niobium, palladium, platinum and vanadium.

Experiments demonstrate that the choice of silver as the additional metal is the most particularly favourable for the solubility of the intermetallic compound at a high temperature, and for obtaining a long FCC phase, since the silver-gold solubility is complete, and since the silver can also dissolve the aluminium.

Innovatively, the invention has attempted to create the pseudo-binary alloy phases Au—Ag—Al as seen in FIG. 1. This diagram shows the various phases in a conventional manner, according, on the x axis, to the concentration of aluminium, i.e. the ratio of aluminium mass to the total alloy mass, and on the y axis, to the temperature, shown here in degrees Celsius. The diagram in FIG. 1 shows the preferred case of a 75% gold mass concentration i.e. the preferred case of an 18 carat alloy.

This is a partial solubility diagram showing the limits of solubility, which are substantially vertical in the diagram, and which separate the phases. Each phase has a defined composition which is different to the next phase. In each of these phases, the atoms are reorganised locally to form precipitates, which are defined compounds of fixed composition.

In order to obtain the desired Al₂Au₅ precipitate, and only that precipitate, it is advisable to remain within a first domain called A in FIG. 1, in which there exist only the alloy elements in FCC form on the one hand, and the Al₂Au₅ precipitates on the other hand. To remain within this domain, the concentration of aluminium must remain less than 2.1%. The range of concentrations to be observed is 0.1% to 2.1% of aluminium to ensure that only Al₂Au₅ is obtained.

A second domain called B in FIG. 1 corresponds to a phase where the Al₂Au₅ and AlAu₂ precipitates co-exist with the alloy elements in FFC form.

The third domain called C in FIG. 1 corresponds to a phase where only the AlAu₂ precipitates co-exist with the alloy elements in FCC form.

The diagram in FIG. 1 shows that, to obtain an alloy in an optimum composition within domain A, one method consists in heating all the alloy elements, to then be within domain D of FIG. 1 which corresponds to a solution heat treatment of the aluminium. A dilution heat treatment at a temperature between the solidus and liquidus delimiting domain D allows a homogeneous solution heat treatment: the gold is in an FCC structure, owing to the additional element or elements chosen, particularly silver, and the FCC structure is stable. A high solubility of aluminium is observed in the phase FCC_A1, at high temperature, in particular at temperatures comprised between 400° C. and 700° C. The additional element or elements also facilitate the solubility of aluminium in gold.

The alloy is then made metastable. The rise in temperature, achieved for example between around 400° C. and 700° C. for the extreme part of domain A and ideally around 650° C., is followed by rapid cooling, such as water hardening or similar. Thus, the aluminium atoms do not have time to reorganise. After a variable period time, but preferably close to 24 hours, the alloy undergoes a structuring tempering treatment, within the temperature range defined by the solvus of domain A for the aluminium concentration concerned. In any event, this structuring tempering treatment does not exceed a temperature of 400° C. During tempering, the Al₂Au₅ precipitates develop and grow. Preferably, the structuring tempering temperature is higher than 200° C. to facilitate the precipitate growth, and also to limit the duration of the heat treatment.

FIG. 2 is a diagram of Vickers hardness on the x axis according to time on the abscissa. It is seen, in the example of FIG. 2 of a structuring tempering treatment at 200° C., that a hardness of more than 250 HV is obtained very quickly, after around 2 hours. This hardness will increase further if the structuring tempering treatment is extended, but asymptomatically, even if maximum hardness is desired, and there is no point in extending the treatment beyond 10 hours, where a hardness of around 280 HV is obtained. FIG. 2 shows, by way of comparison, the level of hardness 150 HV obtained with a conventional 18K or 18 carat gold alloy.

If the structuring tempering treatment is performed at a lower temperature, for example 100° C., a hardness of more than 200 HV will only be obtained after 10 to 15 hours, and the treatment must be extended further to reach a level of around 250 HV.

The Al₂Au₅ precipitate obtained is harder than gold.

It is essential, according to the invention, to favour the presence of the Al₂Au₅ precipitate and preferably to restrict the formation of precipitates comprising only gold and aluminium to this one Al₂Au₅ precipitate, which has the best features, in order to resolve the technical problem of hardening the alloy.

Preferably, in order for the Al₂Au₅ precipitates to develop optimally, the alloy does not include any other metal apart from gold, aluminium and an additional metal, preferably silver, selected to increase intermetallic compound solubility and to lengthen phase D as much as possible in terms of the amplitude of the range of aluminium concentrations.

Returning to some of the aforecited prior art teachings, it is evident that the role of aluminium is not demonstrated.

Document D1 discloses a pink gold.alloy. It cannot be affirmed that aluminium is significant as the alloy hardening agent by analysing the variance of the results of document D1. Copper, like silver, is a hardening element, α=0.05. The effect of copper is to create an order/disorder reaction with gold, with the formation of an AuCu type compound, with increased hardness. The effect of the silver/copper mixture is similar to the effect of copper by itself.

The same is true of document D2, which also discloses a pink gold. The addition of aluminium does not seem to play any particular part therein, which is understandable if the processing method is not carried out for the purpose of forming a single gold and aluminium precipitate which is advantageous for increasing the hardness of the alloy.

Document D3 is also dedicated to fabricating pink gold. This document claims that the alloy remains single phase, which is physically impossible: the teaching of this document cannot be accepted.

Document D4 discloses a pinkish yellow gold. It is impossible to evaluate the specific role of aluminium since all the other elements present in the alloy already contribute to the hardening process. This document comments that with more than 0.4% aluminium, the alloy blackens considerably. This drawback is not visible in the case of the alloy developed according to the invention, even with 2% aluminium content. It should be noted that the experimenter who follows the instructions of document D4 will not obtain Al₂Au₅ precipitate not only because of the added elements such as zinc, which modify the solubility of aluminium, but also because of the low aluminium concentration >0.4%.

Document D5 explains the hardening of the gold (75%)-silver-copper alloy by the well known order/disorder transformation with copper. The composition of 75% gold-12.5% silver-12.5% copper of FIG. 7.12 of document D5 gives an expected hardness of 220 HV, which is much lower than that obtained by the invention.

In short, the fabrication of the aluminium and gold precipitate Al₂Au₅ increases the HV hardness by around 50 HV.

In short, the invention differs from the prior art in that it creates the conditions for developing Al₂Au₅ precipitate within an alloy of suitable composition comprising gold, aluminium and at least one additional metal selected for its ability both to favour a stable FCC structure and to increase the solubility of aluminium in gold, this additional metal preferably being silver.

The optimum weight percent composition is from 0.1% to 2.1% of aluminium, and preferably from 0.5% to 2.1% and at least 75% of gold so as to respect the legal standard for jewellery, and the complement formed by at least one additional metal, which may be completed by a small proportion of at least one other component selected for fluidity and grain size refinement.

The additional metal may also be copper. It is also possible to combine several metals each having the properties that this additional metal is required to have, namely the ability to favour a stable FCC structure on the one hand and the ability to increase the solubility of aluminium in gold on the other hand.

Silver is the best element, and the other metal elements in the list set out above may be added to adjust the colour of the alloy. This list of elements was drawn up so that the elements comprised therein satisfy the condition of increasing the solubility of aluminium in the FCC structure at a high temperature.

In particular, copper is less favourable than silver in satisfying these particular conditions in the presence of gold and aluminium. The use of copper remains possible for reasons of cost, but is considerably less advantageous than silver, and if used should always be combined with silver, while ensuring that the concentration of silver is always higher than the concentration of copper in the alloy.

With the use of additional metals other than silver or copper, for example chosen from among chromium, copper, iron, hafnium, manganese, niobium, palladium, platinum and vanadium (this list is not exhaustive), it should be borne in mind that aluminium could form precipitates with certain of these additional metals, but that preferably it is desired to form Al₂Au₅ precipitates. Thus, in addition to silver and aluminium, preferably only these elements should be used: chromium, copper, iron, hafnium, manganese, niobium, palladium, platinum and vanadium.

Moreover, each new composition with different additional metals requires thorough testing to define the corresponding phase diagrams, which do not exist in the literature, to analyse the precipitates and other intermetallic compound components created within each of the phases, and to check that these compounds do not adversely affect the mechanical properties of the gold alloy. These studies and experiments take a long time and are expensive and cannot be conducted at random. The object thereof is also to determine, on a case by case basis, the aluminium concentration range to be observed to obtain Al₂Au₅ precipitates and preferably only this precipitate.

In short, the invention provides a 3N 18 carat yellow gold-based alloy, characterized in that it is formed of a mixture comprising in weight percent:

at least 75% of gold,

from 0.5% to 2.1% of a second metal chosen for its ability to form precipitates with gold, said second metal being aluminium,

20% to 25%, or preferably 22.4% to 24.5% of at least one additional metal chosen for its ability to favour a high temperature stable FCC structure, and for its ability to increase the solubility of said second metal in gold, and to adjust the colour of the alloy,

0% to 0.5% of one or more components selected for the fluidity and grain size refinement of said alloy,

said mixture further including at least one said precipitate from said second metal and gold, selected from among said precipitates of said second metal with gold to form an intermetallic compound providing said alloy with more than 250 HV hardness so as to improve the structural hardening of said alloy, said selected precipitate being the aluminium and gold precipitate Al₂Au₅.

Advantageously, the second metal is aluminium and the precipitate chosen is the aluminium and gold precipitate Al₂Au₅ which provides an alloy with very good hardness characteristics, i.e. a hardness of more than 250 HV and close to 280 HV. This Al₂Au₅ precipitate also gives the alloy very good resistance during transformation or when machined, since it does not make the alloy brittle.

Preferably, the additional metal is silver, which ensures that the whole of the mixture is properly soluble.

In a particular embodiment, the additional metal is silver, preferably in a weight percent content of 10% to 12.5% of the total, and it is completed by another additional metal, preferably in a weight percent content of 10% to 12.5% of the total, to adjust the colour of the alloy.

Advantageously, at least one said component selected for the fluidity and grain size refinement of said alloy is chosen from among zinc, cobalt or iridium.

The method for obtaining a gold-based alloy, with improved hardness, preferably consists in:

-   -   choosing a second metal for its ability to form precipitates         with gold, said second metal being aluminium;     -   choosing at least one additional metal for its ability to favour         a stable FCC on the one hand, and for its ability to increase         the high temperature solubility of said second metal in gold on         the other hand;     -   creating the conditions for inserting precipitates of said         second metal with gold into an FCC structure resulting from the         solution heat treatment of a mixture of gold, said second metal         and said at least one additional metal;     -   preparing a mixture comprising in weight percent:         -   at least 75% of gold,         -   0.5% to 2.1% of said second metal,         -   20% to 25% of preferably from 22.4% to 24.5% of at least one             said additional metal,         -   0% to 0.5% of one or more components selected for the             fluidity and grain size refinement of said alloy,             said second metal and said additional metal being selected             to obtain from among said precipitates of said second metal             with gold, at least one said particular precipitate of said             second metal with gold to form an intermetallic compound             giving said alloy a hardness of more than 250 HV, said             selected precipitate being the aluminium and gold             precipitate Al₂Au₅;     -   solution heat treating said mixture by heating to between         400° C. and 700° C. and preferably between 650° C. and 700° C.;     -   rapid cooling after said solution heat treatment, preferably in         the form of ambient temperature hardening;     -   performing a structuring tempering treatment after said rapid         cooling at a temperature of between 200 and 400° C. and         preferably between 200° C. and 250° C. to create said at least         one selected precipitate of said second metal with gold which is         the aluminium and gold precipitate Al₂Au₅;     -   allowing the controlled growth of said at least one selected         precipitate by maintaining said structuring tempering treatment         for a sufficient period or time, preferably at least 60 minutes,         to obtain the required hardness;     -   cooling at ambient temperature.

Preferably, the choice of selected precipitates is limited to one precipitate, in this case the aluminium and gold precipitate Al₂Au₅.

Advantageously, the structuring tempering treatment is performed at least 24 hours after the rapid cooling.

Preferably, aluminium is chosen as the second metal, and the aluminium and gold precipitate Al₂Au₅ is chosen as the selected precipitate.

Advantageously, silver is selected as the additional metal.

In a variant, silver is selected as the additional metal and another additional metal having similar characteristics to silver is added, to adjust the colour of the alloy.

Other methods such as mechanosynthethis or PVD may also involve the creation of the aluminium and gold precipitate Al₂Au₅. However, as in the operating modes listed above, the conditions for inserting this very particular precipitate into the face centred cubic structure of the alloy and the conditions for developing this precipitate in order to give the alloy the desired hardness characteristics should be created.

The invention further concerns the use of a precipitate for hardening a gold alloy. According to the invention, this precipitate is an aluminium and gold precipitate Al₂Au₅ for hardening a gold alloy comprising at least 75% of gold, form 0.5% to 2.1% of aluminium and from 20% to 25% or preferably from 22.4% to 24.5% of at least one additional metal selected from among silver and copper and chosen for its ability to favour a stable FCC structure on the one hand, and for its ability to increase the solubility of aluminium in gold on the other hand, and from 0% to 0.5% of one or more components selected for the fluidity and grain size refinement of said gold alloy, said use resulting from the insertion of said aluminium and gold precipitate Al₂Au₅ into an FCC structure resulting from the solution heat treatment of said mixture formed of gold, aluminium, said at least one additional metal, said selected components if said alloy includes any, and aluminium and gold precipitates, said insertion being preferably achievable by the method described above.

The invention further concerns a timepiece, piece of jewellery or dental piece including at least one component made of an alloy of this type. 

1-16. (canceled)
 17. A 3N 18 carat yellow gold-based alloy, comprising in weight percent: at least 75% of gold; from 0.5% to 2.1% of an aluminium metal capable of forming at least one precipitate with gold; from 20% to 25% of at least one first additional metal having an ability to favor a stable face centered cubic (FCC) structure and to increase high temperature solubility of the aluminum metal in the gold; 0% to 0.5% of at least one component affecting fluidity and grain size refinement of the alloy; and at least one aluminium and gold precipitate comprising Al₂Au₅ forming an intermetallic compound which provides the alloy with more than 250 HV hardness so as to improve structural hardening of the alloy.
 18. The alloy according to claim 17, wherein the first additional metal is silver.
 19. The alloy according to claim 18, wherein the first additional metal is 10% to 12.5% weight percent silver, and the alloy further comprises 10% to 12.5% weight percent of a second additional metal of lower concentration than that of the silver.
 20. The alloy according to claim 19, wherein the second additional metal is copper.
 21. The alloy according to claim 17, wherein a content of the at least one first additional metal is between 22.4% and 24.5%.
 22. The alloy according to claim 17, wherein the at least one component affecting fluidity and grain size refinement of the alloy is selected from the group consisting of zinc, cobalt and iridium.
 23. The alloy according to claim 17, comprising, as the only precipitate of aluminium and gold, the aluminium and gold precipitate comprising Al₂Au₅.
 24. A method for obtaining a gold-based alloy with improved hardness, the method comprising: (i) heat treating a mixture comprising: 0.5% to 2.1% of aluminium; at least 75% of gold; 20% to 25% of at least one first additional metal having an ability to favor a stable face centered cubic (FCC) structure and to increase high temperature solubility of the aluminum in the gold; and 0% to 0.5% of at least one component affecting fluidity and grain size refinement, to a temperature between 650° C. and 700° C. to form a heat treated mixture; then (ii) rapidly cooling the heat treated mixture by ambient temperature hardening to form a cooled mixture; (iii) tempering the cooled mixture at a temperature of between 200° and 250° C. to form at least one aluminium and gold precipitate comprising Al₂Au₅, said at least one precipitate being capable of inserting into the FCC structure of the at least one first additional metal and forming an intermetallic compound capable of imparting a desired hardness of more than 250 HV to a resulting gold-based alloy; (iv) maintaining the tempering for at least 60 minutes to grow the intermetallic compound in a controlled manner to obtain the resulting gold-based alloy with the desired hardness; and (v) cooling the resulting gold-based alloy at ambient temperature.
 25. The method according to claim 24, wherein the rapid cooling (ii) is achieved by water hardening.
 26. The method according to claim 24, wherein the at least one precipitate is a single precipitate.
 27. The method according to claim 24, wherein the tempering (iii) occurs for at least 24 hours after the rapid cooling (ii).
 28. The method according to claim 24, wherein the at least one first additional metal is silver.
 29. The method according to claim 24, wherein the mixture further comprises at least one second additional metal in a lower concentration than that of the silver.
 30. The method according to claim 29, wherein the second additional metal is copper.
 31. The method according to claim 24, wherein the mixture comprises between 22.4% and 24.5% of the at least one first additional metal.
 32. A timepiece or a piece of jewellery, comprising at least one component made of the gold-based alloy of claim
 23. 33. A method for hardening an 18 carat gold-based alloy, the method comprising forming at least one precipitate from aluminium and gold comprising Al₂Au₅ which forms an intermetallic compound within a gold-containing alloy and thereby imparts a resulting gold-based alloy with a hardness of more than 250 HV.
 34. The method of claim 33, wherein: the gold-based alloy comprises: at least 75% of gold; 0.5% to 2.1% of aluminium; 20% to 25% of at least one additional metal selected from the group consisting of silver and copper, said additional metal having an ability to favor a stable face centered cubic (FCC) structure and to increase solubility of aluminium in gold; and 0% to 0.5% of at least one component affecting fluidity and grain size refinement of the gold-based alloy; the method further comprises inserting the intermetallic compound into the FCC structure during a solution heat treatment of a mixture comprising the gold, the aluminium, the at least one additional metal, and the at least one component if present; and the at least one additional metal is selected from the group consisting of aluminium, silver, chromium, copper, iron, hafnium, manganese, niobium, palladium, platinum, and vanadium.
 35. The method of claim 34, wherein the at least one additional metal is silver.
 36. The method of claim 35, wherein the alloy further comprises at least one second additional metal having a lower concentration than that of the silver.
 37. The method of claim 36, wherein the at least one second additional metal is copper.
 38. The method of claim 34, wherein the gold-based alloy comprises 22.4% to 24.5% of the at least one first additional metal.
 39. The method of claim 34, where the Al2Au5 is the only aluminium and gold precipitate in the gold-based alloy. 